Industrial vehicles, including forklift trucks known as narrow aisle or reach trucks, include steering systems designed for improved operation of the vehicle. Reach trucks are configured with front load wheels, a left rear drive wheel, and a right rear caster. With the front load wheels commonly fix mounted to straddle arms, vehicle suspension is accommodated by the rear wheel/caster to maintain drive wheel contact on uneven floors. Vehicle suspension is provided by an articulating rear axle or sprung caster assembly.
Whereas reach trucks include a drive wheel and caster, only the drive wheel is directly steered by the steering system. The caster is freely rotated and is provided with a rotational offset to allow a certain amount of self steering when the reach truck changes direction. While the caster rotates during a turning operation of the reach truck, it undergoes some lateral movement or “scrubbing” transverse to the direction of travel. This scrubbing causes the reach truck to temporarily move or lurch in a lateral direction, contrary to the desired direction of rotation, and results in a larger than desired turning radius of the reach truck.
Some steering systems include a mechanical linkage between the drive wheel and the caster. When the drive wheel is turned, the caster is also made to turn by way of the mechanical linkage. This may reduce an amount of delay of the caster to rotate to a new position, but introduces a complexity of mechanical linkage and added routine maintenance. Furthermore, the mechanical linkage requires a substantial amount of room within or underneath the reach truck which reduces the amount of space available to an operator, and may increase a step height of the operator compartment ingress. Increasing the step height introduces ergonomic issues for the operator.
Known mechanical linkage systems approximate the Ackerman design of steering geometry, where a mechanically steered caster is driven via linkage connected to the drive wheel. The mechanical linkage consists of multi bar linkage installed between the drive wheel and the caster. The drive wheel is controlled either hydraulically or electronically by an operator from a steer tiller control. The angle of the caster is then controlled by coordinated movement of the mechanical linkage system. The mechanical linkage systems are only able to approximate the Ackerman steering geometry through a full range of wheel and vehicle turning or steering. Coordination of the steering angles between the drive tire and caster becomes increasingly compromised as the vehicle steering system approaches a minimum or zero turning radius. A minimum turning radius is not attainable in both right and left steering directions with mechanical linkages due to physical constraints of the mechanical linkage.
The steer effort required to operate the mechanical steer system varies depending on the steering angle requested, as a result of changes in moment arm lengths of the linkage. The steer effort is also increased due to friction occurring at the linkage joints. The steering system inefficiencies result in inconsistent steering effort demands placed on the operator and on the steering motor. Increasing the steering force introduces ergonomic issues for the operator. The steering motor must be sized to compensate for the increased steer effort at increase steering requests, and may result in a larger steering motor. A larger steering motor may reduce the amount of space available in the operator compartment, or contribute to an increased step height of the operator compartment ingress.
The present invention addresses these and other problems.